More and more of us are doing it: Marc Mezvinsky and Chelsea Clinton pose during their 2010 interfaith ceremony. Photo: Getty Images

In all the excitement over Princess Kate’s pregnancy, Windsors-watchers may have missed some other important news: Members of the royal family will soon be allowed to marry Catholics.

All 16 Commonwealth nations have consented to change the 300-year-old law that bars British kings and queens from marrying Roman Catholics. The legislation, which UK Prime Minister David Cameron will introduce in Parliament, will amend the Bill of Rights, The Coronation Oath Act of 1688, the 1701 Act of Settlement and the Union With Scotland Act of 1706.

How quaint, you may say. The Brits finally got over that whole Henry VIII issue and decided Catholics aren’t so bad after all.

But it wasn’t so long ago that Catholic-Protestant marriages were taboo in democratic America, too.

In his famous 1954 work “Protestant, Catholic, Jew,” sociologist Will Herberg made the case that America had become a kind of “triple melting pot” in which people, rather than settling into one big stew, had dropped the ethnic part of their identities — but kept the religious part.

So Irish Catholics and Italian Catholics could be in one pot; German Jews and Russian Jews in another. But the religious barriers remained largely stable.

No longer, obviously. Some 45 percent of marriages in the past decade were interfaith. The 2001 American Religious Identification Survey found that 27 percent of Jews, 23 percent of Catholics, 39 percent of Buddhists, 18 percent of Baptists, 21 percent of Muslims and 12 percent of Mormons were married to a spouse with a different religious identification.

Take Amy and Farid (I’ve changed all these names in the interest of privacy). She’s a Christian graduate of Oral Roberts University in Tulsa. He’s a Muslim who immigrated here with his family from Afghanistan at age 3. The two live Upstate, happily raising a toddler.

Lillian, a Lutheran, married Maderu, a Hindu immigrant from India. They live in a suburb of Detroit, where she sings in her church choir and the two give a significant donation to their church every year.

Abbas, an Egyptian Muslim, met his wife, Dorit, an Israeli Jew, while he was working at a hotel in the Sinai desert and she was a travel agent booking tour groups. After a few meetings with her, his friends at the hotel warned him that the Egyptian police were starting to talk about him — plainly suspicious that he was consorting with an Israeli.

He didn’t tell his family about the relationship. “I knew they would be against it. They would lock me at home if they knew.” They moved to America — and when Abbas visited Egypt, he says, “I felt like I went back 100 years.” He could no longer tolerate the bigoted attitudes of his friends and family.

Interfaith couples who’ve lived elsewhere marvel at the level of tolerance in America. And interfaith marriage is actually part of the reason for that tolerance. Research by David Campbell of Notre Dame and Robert Putnam of Harvard shows a clear correlation between being married to a spouse of another religion and one’s feeling toward that religion.

Nicole, whose grandfather was a Southern Baptist preacher, tells me her mom was very concerned about her dating a Muslim. “For her, this was a crack in God’s faith for me to want to marry someone who doesn’t believe Jesus Christ was our lord and savior.”

Nicole’s now-husband and her mother have had long conversations about faith. Now, Nicole says, “My family has accepted him with open arms. All of our views of Islam have changed because we’ve taken the time to educate ourselves. My mom finds herself defending his religion to others.”

There are plenty of downsides to interfaith marriage — it adds tension to the relationship; it results in lower attendance at religious institutions, it makes matters more confusing for kids. But its benefits must be noted as well.

At this time of year, pundits and religious leaders often focus on the December dilemma — Can we have a manger scene in the town square? Can my non-Jewish husband put antlers on his car? Can the cashiers at Target wish everyone a Merry Christmas? But we shouldn’t lose sight of this country’s long tradition of religious toleration. Thanks to intermarriage, it is a tradition that is extending further everyday.

Naomi Schaefer Riley is the author of “’Til Faith Do Us Part: How Interfaith Marriage is Transforming America,” due out in April.